1. Field of Invention
This invention pertains to well screens of the type that are utilized in subterranean drilling for and pumping of natural resources such as oil and gas.
2. Related Art and Other Considerations
A typical oil or gas well comprises an elongated pipe string which includes a plurality of casing or joint assemblies which are normally positioned in the oil or gas bearing portions of a formation. Each casing or joint assembly includes a perforated cylindrical inner or pipe base member which has one or more screen segments covering its perforations so that particulate matter entrained in the fluid will be removed from the fluid before the fluid passes through the perforations and into the inner pipe from whence it is directed axially through the inner pipe to the surface for fluid recovery. The screen segments normally consist of a plurality of longitudinal rod members around which a length of V-shaped wire is spirally wound and welded at every intersection with the rods. Usually, the screen segments are integrally mounted at each of their ends to the pipe base member, such as by welding. In most situations, the screen segments are also covered by outer perforated shroud members which shield them from damage as they are lowered into the formation. On occasion, when a screen segment in an already completed well becomes worn or damaged and is no longer able to provide sand control, it is possible to position a new screen section inside the worn or defective one and this screen section can be supported by its outer perforated shroud member. For corrosion resistance, the various elements are normally fabricated of stainless steel.
Typically, during the installation of each segment of casing which includes a length of well screen, the drilling rig is attached to the pipe base member at an upper portion thereof which is located beyond the end of the screen segment. As the rig is operated to rotate the pipe base member, its rotational and tensile loads will be transferred directly to the pipe base member. This loading causes the pipe base member to deform elastically and plastically. The motion of twisting and stretching of the pipe base member will simultaneously translate the loads into the screen segment through its fittings which are integrally attached to the pipe base member. Since the screen segments are normally much less able than the relatively thick pipe base member to resist such loads, it is sometimes possible for the screen segments to be damaged so that some of their flow slots will be opened up sufficiently to allow particulate matter to pass into the pipe base member. Obviously, such a situation is highly undesirable.
Furthermore, in the usual situation where a screen segment is welded at its ends to a fitting and pipe base member, or to a shroud member, a heat-affected zone will be produced in the screen material around the weld. This zone will have different metallurgical characteristics than the rest of the screen material. For example, the material will be partially annealed, causing it to be softened while also lowering its tensile strength and hardness and increasing the size of the grains in the material. During installation in a well, and especially a horizontal well, tensile, torsion and bending loads will be placed on the pipe base member and these loads can be substantial enough to cause yielding of the pipe base member and screen segment. If the screen segment has a heat-affected zone due to welding, substantially all of the yielding will occur in this small area around the weld rather than be uniformly distributed along the length of the screen segment. This will cause the slots at the ends of the screen to open up enough to cause loss of sand control. However, if there was no welding of the screen and thus no heat-affected zone, the entire screen segment would be of the same tensile strength and would stretch evenly throughout its length. This would widen all of the slots very slightly, about 0.3%, which is the normal yield elongation for stainless steel, and would not cause a loss of sand control In addition to a reduction in tensile strength, a screen segment which has a heat-affected zone will also have a much lower resistance to wear. It has been shown in tests that there is a higher tendency of erosion at the very ends of the screens and this has been attributed to the softer material in the area of the welds. This higher erosion rate would cause premature loss of sand control due to a progressive widening of the screen slots.
One patent which teaches the provision of a mounting for a screen which prevents tensile and torsional stresses from reaching the screen is Sears et al U.S. Pat. No. 4,167,972 in which one end of the screen is welded to the pipe base member while the other end is sealed by means of an elastomeric O-ring member. However, the O-ring is subject to wear and would rapidly deteriorate in the hot temperatures which are often present in deep oil wells. Lilly U.S. Pat. No. 4,378,840 teaches that a ring on one end of a screen segment can be heat shrunk onto the pipe base member, the two being made of different materials, so that one end of the screen can move under thermally induced expansion forces when the assembly is subjected to high temperatures. Similarly, Lilly U.S. Pat. No. 4,416,331 teaches that a screen segment for use in a similar high temperature environment can have rings of a material dissimilar to the pipe base member at each of its ends. The disclosed structure permits the screen segment to remain fixed in position relative to a surrounding gravel pack in the well bore while the underlying pipe base member is free to move axially relative to it due to forces of expansion created by temperature changes. Since the aforementioned Lilly patents only allow relative movement between the pipe base member and screen segment at elevated temperatures they cannot prevent torsional and tensile loads applied to the pipe base member by the drilling rig from being transmitted to the screen segment. Graen U.S. Pat. No. 5,509,483 discloses the use of liquid epoxy to anchor the end fittings of a screen to a stainless steel mandrel.
What is needed, therefore, and an object of the present invention, is apparatus for protecting well screens from damage otherwise occasioned by torsion, tensile and bending forces as a pipe base member or protective shroud member is subjected to such forces by the drilling rig.